1. Field of the Invention
This invention relates to a process for the production of rutile mixed-phase pigments by calcination of mixtures of titanium dioxide or titanium oxide hydroxide with dye-producing compounds of metals, optionally with addition of compounds of antimony, niobium or tungsten, in a directly heated, continuously operated rotary kiln.
2. Background Information
Rutile mixed-phase pigments are known from US-A-3,022,186.
Rutile mixed-phase pigments are colored pigments which are obtained by incorporation of colored transition elements in the crystal lattice of the rutile. Metals such as these are predominantly incorporated as guest component in the host lattice of the rutile, of which the cation radii are of the order of the radius of the titanium (IV) ion. If the valency of the cation to be incorporated differs from that of the titanium ion, namely four, another cation having a different valency is added for statistical valency equalization.
Nickel and chromium rutile mixed-phase pigments have acquired considerable significance for large-scale production. In their case, oxides of relatively high valency, particularly antimony oxide and, less frequently, niobium or tungsten oxide, are incorporated for valency equalization with the chromophoric nickel and chromium oxides (Ullmanns Encyklopadie der technischen Chemie, Verlag Chemie GmbH, Weinheim 1979, 4th Edition, Vol. 18, page 608).
The mixed-phase pigments with a rutile lattice are produced by calcination of homogeneous mixtures of titanium dioxide as the host component and the chromophoric guest components at temperatures of up to 1200.degree. C. Instead of oxidic guest components, heat-sensitive compounds of the metals on which the components are based may also be mixed with titanium dioxide, titanium hydroxide or titanium dioxide hydrate, changing on heating in air into the corresponding oxides, for example, hydrates, hydroxides, carbonates, acetates, nitrates or formates.
The raw material mixtures are usually calcined in rotary kilns which may be either directly or indirectly heated. Where the rotary kilns are directly heated, the starting material is best an aqueous suspension containing the starting components, which largely avoids solids being discharged from the kiln with the smoke gases. Where dry mixtures of the starting components are used, it is better to use indirectly heated kilns because, in their case, the quantity of waste gas can be more or less considerably reduced, depending on the starting materials, so that there is no danger of a high discharge of solids.
Obtaining pigments of high quality presupposes, above all, complete rutilization.
In conventional processes, these conditions are satisfied when long residence times are maintained throughout the entire rotary kiln process, leading to a poor volume-time yield and hence to high process costs.